In surface mining operations (e.g., “strip mining” operations), large areas of land are typically excavated to uncover seams of coal and other minerals. Substantial amounts of the uncovered coal or other minerals are then removed, but quantities of residual coal, rocks and other debris remain exposed to rainwater and air after the surface mining operation has been completed. These exposed quantities of residual coal, rocks and other debris are known as “trailings.”
Surface mine trailings often contain substantial amounts of pyrite, an iron sulfide. When air and water come into contact with the pyrite contained in the surface mine trailings, chemical reaction(s) take place, converting the pyrite to acid and dissolved iron. As the pH of the solution rises, the dissolved iron flocculates or participates.
For example, in one such acid forming reaction, pyrite undergoes oxidation to form a ferric hydroxide flocculent or precipitant known as “Yellowboy” and sulfuric acid. This reaction generates two moles of acidity for each mole of pyrite oxidized, as follows:4FeS2+15O2+14H2O→4Fe(OH)3↓+8H2SO4 Pyrite+Oxygen+Water→“Yellowboy”+Sulfuric Acid
In another acid forming reaction, ferrous iron is converted to ferric iron. It is believed that, in certain areas, the presence of indigenous bacteria may increase the rate at which ferrous iron is converted to ferric iron, as follows:4Fe2++O2+4H+→4Fe3++2H2OFerrous Iron+Oxygen+Acidity→Ferric Iron+WaterThe rate of this reaction is pH dependant. Under severely acidic conditions (pH 2-3), this reaction proceeds relatively slowly in the absence of the needed types of bacteria. However, in the presence of certain bacteria, the pH may be around 5 and this reaction may proceed much more quickly. Thus, this particular reaction is believed to be the rate determining step in the overall acid-generating sequence.
The third type of acid forming reaction that is known to occur is the hydrolysis of iron. This hydrolysis essentially results in the formation of ferric hydroxide and the splitting of water molecules, as follows:4Fe3++12H2O→4Fe(OH)3↓+12H+Ferrous Iron+Water→Ferric Hydroxide+HydrogenIn this reaction, three moles of acidity are formed from each mole of iron that undergoes hydrolysis. The ferric hydroxide formed will precipitate in solid form when the pH is raised above about 3.5. However, most of the ferric hydroxide remains in solution so long as the pH is below about 3.5.
Such acid-forming reactions can result in the formation of acid mine drainage. The term “acid mine drainage” generally refers to run off from mine areas where rainwater has become combined with acid and dissolved metal, such as iron. Acid mine drainage that has a high iron content may have a red or orange color. Due to its acid pH, acid mine drainage may also dissolve other heavy metals it comes into contact with, including copper, lead, or mercury. Acid mine drainage has been known to have deleterious effects on lakes, streams and groundwater and may contaminate drinking water and damage natural habitat.
Various active and passive techniques have been developed for removing acid and dissolved metals from acid mine drainage to render it less damaging to the environment. The passive techniques that have been used to date include; compost wetlands, anaerobic wetlands, open limestone channels, diversion wells, anoxic limestone drains, vertical flow reactors, and microbiological processes (e.g., the Pyrolusite® process). The active techniques that have been heretofore utilized generally involve the addition of alkaline chemicals (e.g., calcium carbonate, sodium hydroxide, sodium bicarbonate or anhydrous ammonia) to raise the pH of acid mine drainage to acceptable levels and to decrease the solubility of dissolved metals, thereby causing the metals to settle out as precipitates (e.g., sludge). The collection and disposal (e.g., trucking away to landfills) of such precipitates (e.g., sludge) can be laborious and costly. Thus, there exists a need in the art for the development of new methods for extracting or isolating metals from the acid mine drainage precipitates and for utilizing such extracted or isolated metals to form articles of manufacture.